Standard - Your classic wheel shape, it's not built to excel in specific scenarios like a narrow or wide wheel, but it's appropriate for most situations. If you're new to skating this is where you want to start.
Smooth - Smooth wheels have less surface friction than treaded wheels and are easier to slide.
Hard Line - OJ's standard issue skate park shape. Hardline wheels come in a conical shape with a wide riding surface for maximum control on any terrain.
Elite Formula - OJ refers to the Elite Formula as their "way too fast" formula. Made from 100% pure urethane, these super durable wheels offer more speed for even more sessions.
99a - Best used in pools and parks with a little more grip than harder wheels, and a little more slide than softer wheels. Ideal for moderately skilled riders who also want to skate street when needed. Hard enough that sliding is a breeze while soft enough for a smooth roll and plenty of grip.
Most urethane skate wheels generally range from 75a-101a, the numbers increase with the hardness of the wheel.